Classification of Internal Combustion Engines PDF

Classification of Internal Combustion Engines 1.1 Introduction The vehicle propulsion is usually obtained by means of engines. An engine is a mechanical device that can convert the chemical energy of a fuel into mechanical energy available at a rotating shaft. The chemical energy is first conver...

Classification of Internal Combustion Engines 1.1 Introduction The vehicle propulsion is usually obtained by means of engines. An engine is a mechanical device that can convert the chemical energy of a fuel into mechanical energy available at a rotating shaft. The chemical energy is first converted into heat through combustion and then the heat is converted into mechanical work by means of a working medium. QLHV ! Wb The subscript ”LHV” stands for lower heating value, which is a value specific to every fuel, measured in laboratory. The subscript ”b” instead, stands for brake. It comes from the way in which we measure the engine’s torque or power: we connect the output shaft to a brake. 1.2 Classification of engines: Internal vs External combustion Engines can be divided into two families: internal combustion engines (ICE) and external combustion engines (ECE). In ICEs the combustion occurs inside of the machine and the fluid which performs the cycle changes its properties. The fluid inside of the cylinders is changed and so it has to be periodically )each cycle) replaced by new fluid (which is usually air and fuel). The cycle is open. In ECEs, the combustion happens in a separate chamber, called burner while the power generation occurs in another element which receives heat from the burner. The working fluid does not have its properties altered (it is not burned) and it does not need to be changed every cycle. Examples of ECEs are the Stirling engine and steam engines. N.B. Turbines have the combustion happening in a burner, but the fluid changes properties (it takes part in the combustion). 10 1.2.1 Classification of ICEs in terms of layout Internal combustion engines can be of two types: rotary or reciprocating. The rotary (Wankel) engine has very limited automotive applications, so it will be briefly described and then it will be set aside. The only automotive application was created by Mazda. It is very compact and there is a rotor that replaces the crank mechanism. The rotor moves in an oval housing. Problems with this engine were the fact that some sides of it were always hot and others always cold. In addition to this, the pollution generated by this engine was pretty high. Reciprocating engine The reciprocating engine is made by some cylinders in which an el- ement called piston can move up and down. A crank-slide mech- anism connects the piston to the shaft. The rod attached to the pis- ton is the connecting rod (con- rod) while the rod connecting the conrod to the shaft is the crank. A shaft connects di↵erent cranks, so it is called crankshaft.The engine is re- ciprocating because when a piston goes up, another goes down. The upper position of the piston is called top dead centre (TDC), while the lowest position of the piston is called bottom dead centre (BDC). The volume that we have at TDC is the minimum volume of the cham- ber an it is sometimes called clear- ance volume, Vc. The volume at BDC is the maximum volume of the chamber. The di↵erence in volume Figure 1.1: Reciprocating engine between maximum and minimum is called displacement (cilindrata), Vd. The total displacement of an engine is Vd times the number of cylinders, i. We can also define a quantity called volumetric compression ratio: rc = VVmax min. The value of the distance between TDC and BDC is called stroke, s = xT DC xBDC ⇡ 2r, where r is the length of the crank. The equality is not always true since the crank might have an o↵set. The diam- 11 eter of the cylinder is called bore, B. So the displacement is: ⇡B 2 Vd = s 4 1.3 Main classification criteria of ICEs ICEs are devices in which a fluid (air and fuel mixture, called charge) generates work by go- ing through a process of combus- tion. The fuel reacts with an oxidizer (air: 79.05% N2 , 20.95% O2 and some other elements like Ar, CO2 ,...). These two reac- tants produce product and heat. The heat increases the pressure in the chamber and said pres- sure acts on the top of the pis- ton. M = Ft r = Fc b is the instantaneous torque. Since the pressure on the piston varies instant by instant, also Ft and M do. The torque produced by the engine is defined as the mean value of the instantaneous work. There are several ways to classify ICEs, but the two main ones are: method of ignition: spark or compression ignition; Figure 1.2: Forces on the piston cycle duration: two or four strokes. 1.3.1 Method of ignition There are two ways of igniting the charge. Spark ignition (SI) In spark ignition the fuel is a low reactivity one, when in vapor form. It is so little reactive that the mixture of fuel and air has to be ignited by means 12  Figure 1.3: Spark ignition of the addition of energy (spark). This allows us to pre-mix air and fuel in a chamber, before the occurrence of the combustion. The charge is then injected into the cylinder and this injection type is called port injection. Inside of the cylinder, the spark plug creates an electrical discharge at the proper moment, igniting the combustion. The temperature increases and the reaction starts. There is then a flame propagation from the ignited part of the charge to the other unburned parts of gas. A very unwelcome event is the self-ignition of parts of gasses that already have a high temperature and pressure and ignite before the flame reaches them. These parts are called end gas and they are located close to the pis- ton. Because of the upward motion of the piston and because of the expan- sion of the already burned gas, their temperature and pressure rise, causing a self-ignition. This generates an abnormal combustion, called knock. The term knock comes from the sound produced by this abnormal combustion. In order to avoid these kinds of abnormal combustion we have to: use the proper fuel (Octane number over 95) limit the maximum pressure by limiting the compression ratio rc = (8 12) The fuel for SI engines is called gasoline. The mass of fuel has to be in an almost constant ratio with the mass of air at all times. if we wanted to reduce the torque (in general, the power) we would need to reduce the mass of fuel that is delivered to the cylinder. In order to keep the mass of fuel to mass of air ratio constant, we would need to reduce the delivered mass of air as well. The device that has the task of controlling the amount of air in that gets delivered to the cylinder is the throttle valve, which is inside the intake line. The valve reduces the mass of air per cycle per cylinder, ma. Basically, it reduces the air mass flow rate: ṁ = maa inc 13  n The term i is the number of cylinders, while nc = m , where m is the number of revolutions per cycle (1 for a two-stroke or 2 for a four-stroke) and n is the engine speed in [rps]. Compression ignition (CI) The fuel used for CI is a very reactive one in vapor form (diesel oil). Since it is very reactive, we do not pre-mix it with air before the combustion. Instead, we push air in the combustion chamber and then the fuel is injected (by devices called injectors) a few moments before the combustion.The high values of T and p activate the reaction of the fuel. The combustion occurs around the fuel and there will be no flame propagation. a small part of the fuel reacts first (pre-mixed combustion) and then the main combustion happens (di↵usive combustion). In CI engines: the fuel has to be able to self-ignite very well (Cetane number); the compression ratio can be larger than the one for SI engines: rc = (14 22) In Diesel engines, since the fuel is injected in the air only before the com- bustion, there is no need to have a constant ratio between mass of fuel and mass of air. So, if we wanted to reduce the engine power, we would just need to reduce the mass of fuel. N.B. Diesel oil is much ore volatile than gasoline. 1.3.2 Cycle duration When we talk about cycle duration, we are referring to the number of strokes of the piston that are necessary in order to complete a cycle. Usually two strokes of the piston make a revolution of the crank. If the cycle is completed in one revolution, the engine is a two-stroke engine. If instead the cycle is completed in four revolutions, the engine is a four-stroke engine. The latter is the most used in automotive applications. Four-stroke engine The cycle of this type of engines is completed in two revolutions (four strokes) and it is composed by six phases (or only five if we put the last two phases together). Let us now examine the six phases, referring to Fig- ure 1.4: Intake phase: it correspond to the first stroke. The piston moves down to BDC and the intake valve is open. Fresh charge is drawn in the cylinder. In order to maximize the mass inducted, the inlet 14  Figure 1.4: Clapeyron diagram for the four-stroke engine valve opens shortly before the stroke starts and closes slightly after it ends. The mixture is drawn inside by the pressure di↵erence between ambient and cylinder. The exhaust valve is closed (6I ! 1); Compression phase: both valves are closed and the piston moves upwards towards TDC. The volume of the chamber decreases and the pressure increases. The phase is shorter than the stroke (1 ! 2); Combustion phase: the combustion is initiated before the piston reaches the TDC and ends with the piston already descending (2 ! 3); Expansion (or power) phase: the piston is pushed down by the extreme pressure in the chamber. this is the phase in which we produce mechanical power (3 ! 4); Exhaust phase: as the piston approaches the BDC, the exhaust valve opens and the spent gas is first pushed out by the pressure di↵erential between the chamber and the outside (blowdown phase, 4 ! 5). When the piston reaches the BDC, the blowdown finishes and the displacement part of the exhaust phase begins: the piston completes a full stroke to displace out the remaining exhaust gas (5 ! 6E ) In Figure 1.5 we can see the timing of the various phases with respect to the crank position. Notice that ↵1 > ↵2. Notice how the combustion starts slightly before we are at TDC and ends after we passed TDC. this is done to maximize torque. The moment at which both intake and exhaust valves are open at the same time is called valve overlap. 15  Figure 1.5: Valve timing diagram for the four-stroke engine Two-stroke engine In the two-stroke engine, the cycle is completed in only one revolution. Instead of the valves, we have some ports, which are opened and closed by the piston covering or uncovering them. This design needs a way to pressurize the charge in order to make it flow inside of the chamber which is at ambient pressure. Let us now examine the six phases: Intake (or charge) phase: the exhaust port is closed and fresh charge enters the cylinder; Compression phase: inlet and outlet ports are closed and the piston goes upwards, reducing the volume and increasing the pressure; Combustion phase: towards the end of the compression phase, the combustion starts; Expansion phase Exhaust phase: the piston goes down because of the expansion and it uncovers the exhaust port (which is higher than the intake one). The gasses start to exit because of the blowdown; Scavenging phase: the piston goes down more and it uncovers the intake port. The fresh charge displaces the exhaust gasses outwards. Since the exhaust port is higher than the inlet one, in order to keep the charge flowing in when the piston goes up again (to start the next cycle) and covers the inlet, we have a controlled charge valve that is open until the exhaust port gets covered by the piston. The scavenging phase coincides with part of the intake phase. 16  Figure 1.6: Valve timing diagram for a two-stroke engine As we said before, we need a device that pressurizes the air that goes into the intake port, so that pi > pe > pe. We can use a mechanical (volumetric) compressor like the Roots compressor or a turbocompressor to do that, but the cheapest and simplest solution is to use the crankcase as a compressor. A one way valve (Reed valve) is inserted on the side of the crankcase. When the piston goes up, the pressure inside of the crankcase is reduced and the valve opens, letting the charge inside. When the piston goes down, the volume of the crankcase is reduced and the valve is closed. The charge is pressurized and gets pushed to the intake port. Two vs four stroke engines The four-stroke engine takes double the revolutions of the two-stroke one to complete a cycle, so its power output should be half of the two-stroke’s one. In reality, the volumetric efficiency of the two-stroke engine is worse than the four-stroke’s one. Even tough the two-stroke has a more compact design and a high regularity of torque, the low efficiency and high emissions (due to the fact that some of the fresh charge is pushed in the exhaust port along the exhaust gasses) are very big disadvantages. The two-stroke is not used in automotive applications, but its compactness makes it very useful for small power units (like ones for chainsaws, small electric generators,...). Big diesel two-stroke engines are used to power large propulsion plants (ships, large electric generators,...) because of their simple structure. Thermal and mechanical drawbacks of these engines are more easily controlled in large engines. 1.4 Other classification criteria of ICEs There are other less relevant methods of classification for ICEs. 17 1.4.1 Air supply ICEs need air to work. We can classify the engines with respect to the way air is delivered to them: naturally aspirated: there is no compressor that delivers air to the engine; supercharged: there is a mechanical compressor that delivers air to the engine. The compressor is moved by the engine; turbocharged: there is a turbocompressor (turbine coupled to a com- pressor) taht delivers air to the engine. The turbine is spun by exhaust gasses coming from the engine. Why would we want to deliver compressed air to the engine? The answer is pretty simple: a larger air density means that we can deliver a larger mass of air. Usually we place an intercooler between compressor and engine in order to reduce the fluid’s temperature thus increasing its density even more. p ⇢= RT 1.4.2 Mixture preparation This classification method ha to be combined with the SI versus CI one. It basically classifies these two categories of engines by the way the mixture is prepares. For SI engines we have: carburettor: used up until the 90’s. The air passe through a Venturi throat, connected to a fuel reservoir (the air scoops up some fuel). The carburettor does not allow for precise control; port fuel injection: an injector places a certain amount of fuel inside of the air just before the inlet valve; direct injection: the injection takes place inside of the cylinder and has to be performed in large advance with respect to the combustion in order to allow the mixture to be formed. For CI engines we have instead: indirect injection: it takes place in a chamber before the entrance of the cylinder and it is valid only for small displacement engines. Efficiency losses are present; direct injection: the injection takes place inside of the chamber. 18 1.4.3 Cooling and engine shape The last two ways to classify ICEs are method of cooling (liquid with a cooling line and a radiator or air cooling) and shape of the engine (in-line, V-engine and opposite cylinder engine). 19 Performance of ICEs When we examine the performance of an ICE, our interest is in its perfor- mance over is whole operating speed range, its efficiency and fuel consump- tion and its emission characteristic. The performance is defined by the torque, the power and the speed of the engine. We can define: The maximum power available at each speed within the useful en- gine operating range. This value is also called full load or wide-open throttle (WOT) power; The range of speeds and powers over which the engine’s operation is satisfactory. Some other useful parameters are the rated speed, which is the speed (of the engine) at which we have the maximum power and the maximum speed, which is the maximum engine speed at which we can go without damaging the engine. Usually the maximum speed is larger than the rated one. Diesel engines have a higher torque over a lower speed range (max Figure 2.1: Full load torque and power of a turbocharged diesel engine 4000-4500 rpm), while SI engines have a lower torque over a wider speed range (max 6000-7000 rpm). 20 2.1 Brake torque and power The engine torque is measured with a dynamometer, also called brake. This device absorbs me- chanical power coming from the en- gine. The engine’s crankshaft is connected to the brake’s rotor. The rotor is somehow coupled with a sta- tor (mechanically, electromagneti- cally,...) which is supported by low friction bearings. The engine drives the rotor, which in turn tries to drive the stator. The torque exerted Figure 2.2: The dynamometer on the stator in order to keep it still, while the rotor spins, is evaluated considering the force measured by a load cell. T = F · b and Pb = T · ! = T · 2⇡n. We can evaluate the power of the engine either using the International Sys- tem (Watts) or by using the thecnical system (horsepower). Notice how the European thecnical system is di↵erent from the American or British one. 1CV = 1P S = 75kgf m · rad s = 735.499W (cavallo vapore); 1HP = 550lbf · f t · rad s = 745.6999W (horsepower) 2.2 Engine work The work is produced inside of the cylinder because of the pressure of the gas that pushes the piston downwards. Pressure data for the gas in the cylinder is measured by a pressure transducer. We can plot the engine cycle in a p-V diagram. The indicated work per cycle (per cylinder) is obtained through integration along the cycle line: I p dV In two-stroke engines, the application of this work formula is pretty straight- forward. In four-stroke engines, instead, we have two di↵erent kind of indi- cated works (areas referred to Figure 2.3): Gross indicated work per cycle: Wig. It is the work delivered to the piston over compression and expansion strokes (area A + area C with sign, for NA engines); Net indicated work per cycle: Win. It is the work delivered to the piston in the entire cycle (area A - area B, for NA engines) 21  Figure 2.3: Work cycles in engines The sum of A and C is the work transfer between the piston and the cylinder gasses during intake and exhaust phases and it is called pumping work. This work is negative in naturally aspirated engines, since the intake pressure is lower than the exhaust pressure. In supercharged or turbocharged engines, it is possible to have pi > pe and so we can have the pumping work as a positive quantity. We cannot use this configuration with SC and TC engines since due to the EGR (exhaust gas recirculation) regulations we need to have pi < pe. This regulation is used to reduce the N Ox gasses released into the atmosphere by the combustion in the chamber (oxygen combines with nitrogen in the chamber due to high temperatures). Basically we send a part of the exhaust gasses into the intake port so that the charge will be diluted by them. This creates a slower combustion with lower temperatures, thus reducing the chance of N Ox formation. We send these gasses via pressure di↵erential, hence the need to have pi < pe. 2.2.1 Indicated power We can define the indicated power as: Wi · i · n Pi = m The indicated power is the sum of the brake power and of the power that is lost due to friction in the engine, driving the engine accessories and the pumping power (only when we consider the gross indicated power). So, Pig = Pb + Pf 2.3 Efficiency definitions There are several useful values that we can consider. 22 Mechanical efficiency It is the ratio of the brake power (useful power) and of the gross indicated P power: ⌘m = pPigb = 1 Pigf Typical mechanical efficiencies are around 90% at low speeds and around 75% at maximum rated speed. In idle conditions it is equal to zero. Mean E↵ective Pressure It is a way to compare engines regardless of their displacements (bigger displacement means larger torque). It is defined as: W mep = Vd Pi m We can have the indicated mep and the brake mep: imep = iVd n and Pb m T 2⇡m bmep = iVd n= iVd.The imep represent the area of the cycle (summing areas with sign). Even tough it is not a pressure physically speaking, it has the units of a pressure mJ3 = P a. Specific fuel consumption and efficiency n The specific fuel consumption is the fuel flow rate ṁf = mf i m divided by m˙f m˙f the power output. We can have: isf c = Pi and bsf c = Pb , measured in g/kW h. Low values of sfc are desirable. We can define the brake fuel conversion efficiency, which is the value that tells us how much of the fuel’s energy is actually converted into power: Pb Wb ⌘f = = ṁf QLHV mf QLHV The term QLHV represents the lower heating value of the fuel which is a measure of its energy. Notice that bsf c = ⌘f Q1LHV. We can also have an indicated fuel conversion efficiency: Wi ⌘i = mf QLHV Specific power Pb,max iVd 23 2.3.1 Air/Fuel and Fuel/Air ratio One of the most important parameters when it comes to defining the engine operating conditions is the Air/Fuel ratio. It is defined as: ṁa ma ↵= = ṁf mf We are usually interested in normalizing this ratio with respect to the stoi- chiometric ratio, which is the Air/Fuel ratio in stoichiometric conditions (we have the amount of air that we need in order to burn a kilogram of fuel): ↵ = ↵st Considering instead the Fuel/Air ratio, we have: 1/↵ = 1/↵st We can evaluate the stoichiometric ratio by considering the reaction between fuel and oxygen: b b b Ca Hb + (a + )(O2 + N2 ) ! aCO2 + H2 O + (a + ) N2 4 2 4 nN2 79 where = nO2 = 21 = 3.77. So we can define the stoichiometric ratio: 2a(a + 4b ) MN + 2(a + 4b )MO ↵st = aMC + bMH For both gasoline and diesel, values of this ratio are around 14.6. We can define the energy content of a quantity of fuel by using mf ·QLHV. If we are in stoichiometric conditions, we can define the so called energy parameter: QLHV ↵st. Notice how, for instance, hydrogen has a very high lower heating value but also has an high stoichiometric ratio. This means that its energy parameter is not that much higher than the one of other fuels. 2.3.2 Volumetric efficiency The intake system restricts the amount of air that can be inducted inside of the engine. We use a parameter called volumetric efficiency in order to measure how e↵ective the engine is at removing burnt gasses and at inducting new charge. This parameter is only used in four-stroke engines since they have a distinct induction (intake) phase. It is defined as the the actual mass of air inducted divided by a reference mass of air: ma ma ṁa v = = = ma,ref ⇢a Vd ⇢a iVd n2 24 The reference air density that we should consider depends from the type of air supply. In fact, for naturally aspirated engines, we choose the ambient air density while for supercharged or turbocharged engines we choose the air density of the air in the intake manifold. In this way we can see how much air we have in the cylinder with respect to the air that we have in the manifold (we measure losses inside the induction circuit). 2.3.3 Relationships between performance parameters ima n v ⇢a iVd n/2 Pb = ⌘f ṁa QLHV = ⌘f QLHV = ⌘f QLHV (2.1) ↵ m ↵ Wb Pb bmep = = (2.2) Vd iVd n/2 By combining Equation 2.1 and Equation 2.2, we get v ⇢a QLHV bmep = ⌘f (2.3) ↵ We can also say that Pb = T ! = T · 2⇡n (2.4) And so, 4⇡T bmap = (2.5) iVd From these equations we can see that in order to increase the power, we have to increase ⌘f or v or by having fuels with an higher QLHV. Also, an high volumetric efficiency means low pressure losses. In order to increase the power we can increase the speed. 2.4 Performance maps The wide-open throttle condition is not always reached (almost never reached) so we need to be able to understand how much torque we have in other conditions. When the throttle is not fully open, the max- imum torque reached is lower and the peak is situated at lower speeds. Obviously the speed plays its part (lower torque at higher speeds). No- tice how the throttle valve is never fully closed. This is because we need some fuel to be delivered in order to keep the engine running at idle con- Figure 2.4: Influence of throat angle dition. on torque 25  Figure 2.5: Fuel consumption efficiency map 2.4.1 Efficiency maps There are several efficiency maps that we can use, one for each efficiency type. We have an important re- lationship which is: ⌘f = ⌘i ⌘m. The efficiency that we are mostly interested in is the fuel consumption efficiency (Figure 2.5). We can notice an ”eye” in the map, in which we can find the maximum of the efficiency. Along with efficiency maps, we have fuel consumption maps, which indicate the fuel consumption of our engine in a bmep-engine speed plane. In the bsfc map we can see a maximum at low-medium engine speed and medium-high loads. 2.4.2 Emission maps Lastly, we have emission maps. We have maps for most of the gasses that are emitted by the engine (HC, CO, N Ox ,...) 26